Volumetric light rendering a hard cut-off in the scatter rays
The problem was the camera clipping distance was less than the size of the volume.
By reducing the size of the volume so that the far side of the volume is within camera clipping distance the artifact (mostly disappeared)

A second problem I then had was the top of the volume I had given a surface shader (and a separate material) which caused lesser artifacts on top of any objects. Removing the surface shader fixed this.